SUMMARY / ABSTRACT The etiology of ischemic brain injury remains elusive. Studies of the mechanism(s) of injury during hypoxia and ischemia have faced three major limitations in the past. First, it has been difficult to measure some of the putative etiological factors with adequate sensitivity and temporal resolution. Either the sensors do not exist or they do not function well in the hypoxic environment in live tissue. Second, the experimental conditions have not mimicked brain ischemia effectively. Finally, studies of the effects of hypoxia in intact animals have often been limited to analyses of behavior and the anatomical extent of tissue damage after the injury rather than focusing on the activity of mediators of injury and repair as they evolve during the actual ischemic insult. As a result, the mediators of injury in reduced preparations (e.g., glutamate, lactate) have not been studied extensively in intact animals. To address these limitations, we have the following aims: 1: Develop and characterize novel electrochemical biosensors for glutamate and lactate based on metal oxide technology so that they work in a low oxygen environment. 2: Test the biosensors in the middle cerebral artery occlusion model of ischemic brain injury in intact animals to determine the temporal profile of changes in lactate and glutamate. The new enzyme sensor design will expand the arsenal of accessible electrochemical probes for in vivo measurement of glutamate and lactate, and provide new methods for studying the neurobiology of these neurotransmitters in hypoxic conditions. This research will facilitate study and further fundamental understanding of glutamate and lactate neurotransmission in a variety of neurological disorders in which oxygen is a restrictive factor.